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56 Manual of basic techniques for a health laboratory
Eyepiece
Magnification
The magnifying power of the eyepiece is marked on it (Fig. 3.6):
— a ¥ 5 eyepiece magnifies the image produced by the objective five times;
— a ¥ 10 eyepiece magnifies the image 10 times.
If the object is magnified 40 times by the ¥ 40 objective, then by five times by the
¥ 5 eyepiece, the total magnification is: 5 ¥ 40 = 200. To calculate the total
magnification of the object observed, multiply the magnifying power of the objective
by that of the eyepiece. Microscopes used in medical laboratories have a magnifying
power of between ¥ 50 and ¥ 1000.
Certain eyepieces have a calibrated graticule. These eyepieces are used to measure
the size of an object under the microscope (e.g. protozoan cysts).
Fig. 3.6 An eyepiece
Binocular microscopes
Binocular microscopes (two eyepieces but using only one objective at a time) are
generally recommended. They are less tiring for the eyes than monocular microscopes
when long examinations have to be made. Electric illumination is, however,
essential for using the ¥ 100 objective.
Illumination system
Light source
An electric light source is preferable, since it is easy to adjust. It is provided either
by a lamp built into the microscope beneath the stage, or by an external lamp
placed in front of the microscope.
Mirror
The mirror reflects rays from the light source onto the object. One side has a plane
surface, the other a concave surface (Fig. 3.7). The concave side forms a low-power
condenser and is not intended to be used if the microscope already has a condenser.
Fig. 3.7 A microscope mirror
Condenser
The condenser (Fig. 3.8) brings the rays of light to a common focus on the object
to be examined. It is situated between the mirror and the stage.
The condenser can be raised (maximum illumination) and lowered (minimum illumination).
It must be centred and adjusted correctly.
Fig. 3.8 A condenser
Diaphragm
The diaphragm (Fig. 3.9), which is inside the condenser, is used to reduce or
increase the angle and therefore also the amount of light that passes into the
condenser.
The wider the diaphragm the greater the numerical aperture and the smaller the
detail seen. But the contrast is correspondingly diminished.